1. Field of the Invention
The present invention relates to a magnetic circuit component molding device for molding magnetic circuit components comprising a permanent magnet and magnetic yoke such as used in speakers and motors and, more particularly, to a device for integrally molding a center magnetic yoke within a ring-shaped magnet.
2. Description of the Prior Art
Magnetic circuit components using ring-shaped magnets are primarily used for rotors in electric motors. In most cases, these ring-shaped magnets consist of three to six curved magnet pieces that are applied to the magnetic yoke of the shaft using an adhesive.
That part of this magnetic circuit component that is inside the ring-shaped magnet and in contact with the inside circumference of the ring-shaped magnet molding and pass of the magnetic flux flowing out from the ring-shaped magnet is referred to below as the "center magnetic yoke." The magnetic yoke of the rotating shaft of an electric motor as referred to above is a type of center magnetic yoke.
Because of the way ring-shaped magnets are used, radially oriented magnets manufactured with easy magnetization axes oriented in the radial direction are used as ring-shaped sintered magnets. Barium ferrite magnets, however, tend to crack during sintering because of the difference in the thermal expansion coefficients in the orientation direction (radial) and the direction perpendicular thereto (circumference). It is difficult to manufacture radially oriented magnets without cracking, and crack-free radially oriented magnets are therefore rarely used. Some are used with knowledge of cracks, but in this case adhesive or some other means is used to securely fasten the magnets to the center magnetic yoke or other component.
Developed some years ago, neodymium-iron-boron magnets offer higher mechanical strength than the above barium ferrite magnets. This high mechanical strength prevents cracking caused by the internal stress created by thermal contraction differences during sintering, and thereby makes it possible to manufacture crack-free radially oriented magnets. Because these magnets are sintered bodies, however, uneven condensation can occur during sintering and the dimensional precision of the ring is poor. It is therefore necessary to grind the sintered body to the required dimensional precision before combining the sintered bodies with the center magnetic yoke.
As described above, adhesive methods are usually used with the above magnets. An integral molding method for molding the ring-shaped magnet and center magnetic yoke together in a body has been proposed in, for example, Japanese Patent Laid-Open Publication No. H4-115699. This method uses a die manufactured to the outside dimensions of the ring-shaped magnet, absorbing the pressure applied from inside the ring-shaped magnet with the die.
A more specific application of this proposed integral molding method is described below with reference to FIG. 8. In FIG. 8, the ring-shaped magnet 1 is fit inside the magnet support 22 in the middle of the top die 21, and is supported by the bottom die 23. The top die 21 and bottom die 23 are connected with a pin 24 after the ring-shaped magnet 1 is loaded. In this method, the center magnetic yoke is formed with a mix or blend of magnetic powder and binder, i.e., a powder material Cm commonly called "compound." This powder material Cm is loaded inside the top die 21 and integrally molded to the ring-shaped magnet 1 by pressurized molding using a press 26. The pin 24 is then removed, the bottom die 23 is removed, the integral molding is pushed down and out of the top die 21 by the punch 26. Thus formed integral molding is, then, heat-cured to finish the magnetic circuit component of integrally molded ring-shaped magnet 1 and center magnetic yoke.
As described above, due to an uneven condensation during the sintering process, the dimensional precision of the ring-shaped magnet is poor with these conventional sintered magnets. This makes it necessary to grind the inside circumference of the ring-shaped magnet so that when the ring-shaped magnet is combined with the center magnetic yoke a gap creating magnetic resistance is not formed between the mating surfaces. Grinding an outside surface is relatively simple and low cost. Grinding the inside circumference surface, however, requires the frequent changing of small grindstones. This greatly increases the unit cost of the magnets, which is obviously a major problem. It is also difficult to assure dimensional precision in the outside circumference of the ring-shaped magnet and coaxial alignment of the ring-shaped magnet and center magnetic yoke axes when these are combined by adhesive, leading to the need to eliminate this adhesive process.
The manufacturing method that integrally molds the ring-shaped magnet with the inside center magnetic yoke can, however, eliminate this adhesive step. On the other hand, this method forms the inside diameter of the magnet support member of the top die to the outside diameter of the ring-shaped magnet, and the ring-shaped magnet is fit to the die. When pressure is applied to the magnetic yoke material with this method, pressure is applied to the ring-shaped magnet radially from the magnetic yoke material inside the ring-shaped magnet. When the pressure in the circumferential direction of the ring-shaped magnet created by this radial pressure is less than the tensile strength of the magnet, the ring-shaped magnet can withstand the pressure. Cracking occurs, however, when this pressure exceeds the tensile strength of the ring-shaped magnet. Cracking does not occur when the inside diameter of the top die magnet support member matches the outside diameter of the ring-shaped magnet because the pressure acting on the ring-shaped magnet is sustained by the top die 21.
When this method is applied to integrally mold ring-shaped magnets of a neodymium-iron-boron (Nd--Fe--B) sintered body, however, cracking occurs in 80-90% of the moldings. The outside circumference of the ring-shaped magnet is manufactured with a centerless process with an outside diameter dimensional precision of .+-.20 .mu.m. The inside diameter dimension of the top die magnet support member is processed to match the maximum outside diameter of the ring-shaped magnets. As a result, a gap frequently occurs between the outside diameter of the ring-shaped magnet and the inside diameter of the support, and this gap is thought to permit cracking.
This gap is on the order of 0-40 .mu.m. With a Nd--Fe--B sintered magnet, the tensile strength is a low 8.0 kgf/mm.sup.2 with a Young's modulus of 1.6.times.10.sup.4 kgf/mm.sup.2. The elongation at which destruction occurs due to tension is therefore a low 0.5.times.10.sup.-3. With a 25 mm diameter ring-shaped magnet, cracking therefore occurs when the diameter increases only 12.5 .mu.m.
In mass production, it is technologically difficult and very costly to further increase the processing precision, and we may conclude that preventing cracking with this integral molding method is difficult if not practically impossible. Development of an integral molding method with a high yield and low incidence of cracking is thus desirable.
Furthermore, an inside pressure acting in the radial direction to force the ring-shaped magnet 1 to the outside is increasingly generated while the compound Cm is compressed by the punch 26, resulting in the cracking of the magnet 1. To solve this problem, a conventional top die 21 comprised of plural die units each provided with a hydraulic cylinder is proposed. Each of these hydraulic cylinders is then driven simultaneously by means of a complex hydraulic control system outside the mold. This increases the space needed for the press mold is accordingly greater.
The object of the present invention is therefore to provide a production method and die mold for magnetic circuit components for integrally molding the ring-shaped magnet with the center magnetic yoke placed inside the ring-shaped magnet without causing cracks in the ring-shaped magnet.